After a period of use, internal combustion engines frequently require substantial overhauling and rebuilding to correct problems resulting from cylinder wear, glazing, and other conditions causing improper tolerances between the piston rings and cylinder walls. These conditions can be corrected during overhaul of an engine by using appropriate honing, glaze-breaking, or similar tools to recondition the cylinder walls.
Large machine shops may utilize rather expensive boring machines to precisely recondition cylinder walls. Such machines are very costly, however, and beyond the means of many smaller garages and "backyard mechanics" who therefore must turn to less expensive alternatives. One popular alternative is the use of a honing tool which can be inserted into the chuck of a conventional hand-held electric drill. Such honing tools have been designed to include, for example, four honing stones mounted on a frame which resiliently urges the honing stones outwardly to engage the cylinder wall. Such devices accommodate the reasonable variations in the angle at which the drill is being held with respect to the cylinder axis, thereby producing a consistent and substantially true cylindrical shape for the new cylinder wall.
Use of such hand tools, however, produces problems of its own. Although the hand held honing tool is substantially cheaper than expensive automated machinery, the cost of such a tool is not insignificant and certainly the honing stones are not disposable. A mechanic must therefore be careful when using the tool in the cylinder bore not to extend the tool beyond the opposite end of the cylinder bore. Frequently there are metal castings closely within the area just beyond the end of the cylinder bores of an engine block, and if the honing stones come into contact with such metal castings or protrusions, the stones may be easily damaged or destroyed. In many engines, such protrusions are very close to the end of the cylinder, for example, within the range of about one-half inch to two inches beyond the end of the cylinder bore.
It is frequently difficult for the mechanic to view these protrusions during operation of the honing device. To avoid the rather expensive consequence of ruining the honing stones, mechanics therefore tend to shy away from the opposite end of the cylinder, effectively producing a tapered cylinder wall. The taper often may be in the range of 0.005-0.008 inches (0.15-0.20 mm). A taper of this dimension will cause a substantial detrimental effect on the fit of the rings, reducing compression and causing oil blow-by. An engine having cylinders with a taper in the range of 0.005-0.008 inches, for example, may use about one quart of oil every 1,000 miles, and may have a loss in gas mileage of approximately 1-3 miles per gallon. Furthermore, the loss of compression and oil blow-by contribute to pollution of the environment by increasing exhaust emissions.